In the textile processing industry, .alpha.-amylases are traditionally used as auxiliaries in the desizing process to facilitate the removal of starch-containing size which has served as a protective coating on yarns during weaving.
Complete removal of the size coating after weaving is important to ensure optimum results in the subsequent processes, in which the fabric is generally scoured, bleached and dyed. Enzymatic starch break-down is preferred because it does not involve any harmful effect on the fibre material.
In order to reduce processing cost and increase mill throughput, the desizing processing is sometimes combined with the scouring and bleaching steps. In such cases, non-enzymatic auxiliaries such as alkali or oxidation agents are typically used to break down the starch, because traditional .alpha.-amylases are not very compatible with high pH levels and bleaching agents. Alternatively, unrealistic high amounts of .alpha.-amylases, optionally in protected form, have to be used for such combined processes. The non-enzymatic breakdown of the starch size does lead to some fibre damage because of the rather aggressive chemicals used.
Accordingly, it would be desirable to use .alpha.-amylase enzymes having an improved resistance towards or being compatible with oxidation (bleaching) agents at elevated pH, in order to retain the advantages of enzymatic size break down in a time-saving and environmentally desirable simultaneous desizing and bleaching process.
U.S. Pat. No. 4,643,736 discloses a process for desizing and bleaching performed in a single operation, in which sodium chlorite is used in combination with a strong base, a surface active agent, an activator, and an amylolytic enzyme. However, the use of sodium chlorite is undesirable from an environmental point of view.
EP 119 920 discloses a process for simultaneous desizing and bleaching, in which sodium tetraborate decahydrate is used as buffer in a bath containing hydrogen peroxide, a sequestering agent, an amylase and a surfactant.
In both of the processes described in the above patent publications a relatively high amount of .alpha.-amylase is used, presumably in order to compensate for the low oxidation stability of the .alpha.-amylases used.
PCT/DK93/00230 discloses .alpha.-amylase mutants having improved oxidation stability. The mutants are indicated to be useful for desizing, but their use in a combined desizing and bleaching process is not mentioned.